1. Field of the Invention
This invention relates to a process for removing metallic-salt, ash-producing residues from poly(vinyl alcohol) polymers. More particularly, it relates to a slurry process which utilizes an ion-exchange resin to separate the ash-producing residues from the polymer.
2. Description of Related Art
Poly(vinyl alcohol) homopolymer and poly(vinyl alcohol) coploymers, (collectively referred to hereinafter as poly(vinyl alcohol) polymer, or by the abbreviation PVA) are produced from the corresponding poly(vinyl acetate) homopolymer and poly(vinyl acetate) copolymers by acid or base catalysis. The process may be catalyzed by either strong acids or strong bases, and is known as saponification, hydrolysis or alcoholysis, depending on the catalyst. Base catalysis is generally preferred due to greater efficiency and reaction rate. Unfortunately, any unremoved remaining base tends to destabilize the PVA.
Bases are difficult to wash out of PVA completely, since they tend to be quite strongly absorbed. Base instability may be countered by neutralization of any trace base remaining using weak acids. This produces weakly basic salts such as sodium acetate which are somewhat more amenable to washing out. Such salts themselves, while less destabilizing are, however, still somewhat destabilizing, particularly vis a vis recent more demanding uses of PVA. In addition they are ash precursors, in that ash results on subsequent thermal processing of PVA derived products. (Ash, as used here, is the oxide, hydroxide or related complex that results from heating such salts).
Acid catalysis, does not, of course, result in metallic residues. However, trace acid also destabilizes PVA. Trace acid is also more difficult to wash out than salts. Bases may be used to neutralize the trace acid. Once again however, metal salts will remain.
Residual salts, which can amount to up to 1 weight percent, in addition to producing thermal and color stability problems, albeit less than base itself, lead to ash which has its own associated problems. In transparent PVA derived products, such as butyrated poly(vinyl alcohol) or poly(vinyl butyral), which is used as a windshield interlayer, ash produces haze at levels above about 0.2 weight percent, and even below this level, can affect weatherability. There is therefore a strong incentive to reduce ash-producing residues in PVA to very low levels.
The water-soluble ash precursors, the metal salts, can be removed by extensive washing with water or other solvents, as taught in U.S. Pat. No. 2,642,419. However low salt levels are only achieved by the use of large amounts of solvent, which is inefficient.
Ion-exchange resins (IERs) are known in the removal of metal salts from PVA. However, their use has involved solution of the PVA. U.S. Pat. No. 2,940,948 discloses a process where the PVA and salts are dissolved in water, and the metal cations removed with an acidic, cationic IER. Solid PVA must then be regenerated from the PVA solution.
It would be highly desirable to have a process to remove ash-precursor metal salts by an economical process which involved neither use of large amounts of solvent, nor the need to dissolve and reprecipitate the PVA.